Carbamic acid esters (urethanes) are compounds that are widely used in applications such as polyurethane foam, surface coatings, elastomers, paints and adhesives, and are industrially extremely useful. In addition, carbamic acid esters are also useful as raw materials for producing isocyanates without using phosgene.
Industrial production of isocyanates mainly uses a reaction between an amine compound and phosgene (“phosgene method”), and nearly the entire amount of isocyanates produced worldwide is produced using the phosgene method. However, the phosgene method has numerous problems.
Firstly, a large amount of phosgene is used as a raw material. Phosgene is an extremely highly toxic substance, its handling requires special precautions to prevent handlers from being exposed, and special measures are also required to detoxify waste.
Secondly, since a large amount of highly corrosive hydrogen chloride is produced as a by-product of the phosgene method, in addition to requiring a process for detoxifying this hydrogen chloride, since hydrolysable salts are frequently contained in the isocyanates produced, in the case of using isocyanates produced according to the phosgene method, there are cases in which they have a detrimental effect on the weather resistance and heat resistance of polyurethane products.
In consideration of these factors, there is a need for a process for producing isocyanate compounds that does not use phosgene. One process that has been proposed for the production of isocyanates without using phosgene involves thermal decomposition of carbamic acid ester. Isocyanates and hydroxy compounds have long been known to be obtained from thermal decomposition of carbamic acid esters (see, for example, Non-Patent Document 1: Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870). The basis reaction thereof is indicated by the following formula:R(NHCOOR′)x→R(NCO)x+xR′OH  (1)(wherein R represents an organic residue having a valence of x, R′ represents a monovalent organic residue, and x represents an integer of 1 or more).
In this manner, although carbamic acid esters are industrially useful compounds, since carbamic acid esters easily form hydrogen bonds between molecules from ester groups forming the carbamic acid ester, they frequently have a high melting point. Typically, in the case of using a substance industrially, operations such as those for transferring that substance or storing that substance in a storage tank for a fixed period of time are required. In the transfer of a carbamic acid ester having a high melting point, a solid carbamic acid ester, for example, is crushed or treated with a vehicle for processing into the pellets and the like prior to transfer, or the carbamic acid ester is liquefied prior to transfer by heating to a temperature higher than the melting point of the carbamic acid ester. However, in the case of transferring the solid carbamic ester that has been treated with the vehicle for processing into the pellets, there is a need for a complex apparatus to ensure stable transfer of a fixed amount of carbamic acid ester or the need for a process for maintaining the form of the carbamic acid ester within a certain range in cases of the risk of clogging of the transfer line or frequent fluctuations in the form of the carbamic acid ester. On the other hand, in the case of transferring carbamic acid ester in the form of a liquid by heating, it is necessary to heat to a temperature higher than the melting point of the carbamic acid ester (for example, 200° C.) in consideration of preventing solidification during transfer. In the case of holding a carbamic acid ester under such high temperatures, undesirable side reactions may occur that cause a decrease in the yield of the carbamic acid ester. Examples of such side reactions may include the reactions of the following formulas (2) and (3) that occur due to isocyanate formed by the occurrence of a thermal decomposition reaction of carbamic acid ester as shown in formula (1) above, and the thermal denaturation reaction of carbamic acid ester as shown in the following formula (4) (see Non-Patent Document 1 and Non-Patent Document 2).
(wherein each of R and R′ independently represents an organic group such as a aliphatic group or alicyclic group).
These side reactions not only lead to a decrease in the yield of carbamic acid ester, but in the case of handling carbamic acid esters in particular, there may also be precipitation of polymeric solids resulting in clogging of transfer lines or accumulation in storage tanks.
Several methods have been proposed to solve these problems.
Patent Document 1 discloses a method for storing or transporting aromatic urethane (carbamic acid ester equivalent to the product of a reaction between an aromatic isocyanate and a hydroxy compound) in the presence of an organic solvent. Although this method is characterized by the use of from 1 to 10 times the weight, based on the aromatic urethane, of an organic solvent that is inert with respect to the urethane and the isocyanate corresponding to that urethane, in this method, a decrease in the urethane cannot be inhibited, and a large amount of substances having unknown structures are produced.
In addition, Patent Document 2 discloses a method for storing an aromatic urethane solution by using 1,4-dioxane as a solvent for dissolving the urethane. However, in this method, since an equivalent amount (for example, 20 times the weight) of 1,4-dioxane must be used with respect to the urethane, this method had the problem of resulting in a decrease in the storage efficiency of the urethane.
In this manner, methods used to transfer or store carbamic acid esters without causing denaturation thereof still have problems remaining.
On the other hand, various processes have been proposed thus far for the production of carbamic acid esters.
According to the description of Patent Document 3, an aliphatic diurethane and/or alicyclic diurethane and/or aliphatic polyurethane and/or alicyclic polyurethane are obtained by reacting an aliphatic primary diamine and/or alicyclic primary diamine and/or aliphatic primary polyamine and/or alicyclic primary polyamine with O-alkylcarbamate in the presence of an alcohol and in the presence or absence of a catalyst at 160 to 300° C. at a ratio of amine NH2 group: carbamate:alcohol of 1:0.8 to 10:0.25 to 50, followed by removing the ammonia formed as necessary.
In addition, according to Patent Document 4, an aryl diurethane and/or aryl polyurethane is produced by reacting an aromatic primary amine and/or aromatic primary polyamine with O-alkylcarbamate in the presence or absence of a catalyst and in the presence or absence of urea and alcohol to form an aryl diurethane and/or aryl polyurethane followed by removing the ammonia formed as necessary.
Other publications contain descriptions relating to partial substitution of urea and/or diamine by a carbonyl-containing compound such as N-substituted carbamate and/or dialkyl carbonate, or mono-substituted urea, di-substituted urea, mono-substituted polyurea or di-substituted polyurea (see Patent Document 5, Patent Document 6, Patent Document 7, Patent Document 8 and Patent Document 9). Patent Document 10 describes a process for producing aliphatic O-arylurethane by reacting a (cyclic) aliphatic polyamine with urea and an aromatic hydroxy compound.
In addition, according to Patent Document 11, a process is disclosed for producing a carbamic acid ester from an amine compound and dimethyl carbonate. This process reacts an amine compound and dimethyl carbonate in the presence of Lewis acid catalyst, lead, titanium, zirconium catalyst or alkaline catalyst and the like.
In this manner, although various methods are known for producing carbamic acid esters, at the time of using these carbamic acid ester, an operation is required for recovering the carbamic acid ester from a mixture containing the carbamic acid ester produced according to these methods. Several methods have been disclosed for recovering carbamic acid esters.
Patent Document 12 discloses a method distilling one or more types of diurethanes in the presence of a low boiling point alcohol having an alkyl group having 1 to 6 carbon atoms or an alicyclic hydrocarbon group having 5 or 6 carbon atoms. However, this method also had the problem of solid substances remaining in the distillation apparatus.
In addition, Patent Document 13 describes a method for distillative separation of an unreacted amine compound and alcohol from a reaction liquid obtained by reacting a carbonic acid ester and an amine compound. However, since a solution mainly containing carbamic acid ester is heated in the bottom of a distillation column during the time the distillative separation operation is being carried out, a thermal denaturation reaction like that described above may occur according to this method as well, thereby preventing the obtaining of an adequate recovery rate. In addition, Patent Document 14 describes a process for producing isocyanate by thermal decomposition of a carbamic acid ester after having synthesized the carbamic acid ester by reacting a diamine and dimethyl carbonate. In this process, although the carbamic acid ester is isolated by distillative purification, this distillative purification is preferably carried out in the presence of an inert solvent having a boiling point at least 10° C. lower than the carbamic acid ester. In this distillative purification method, since the carbamic acid ester is heated in the bottom of a distillation column in the same manner as the previously described method, the recovery rate cannot be said to be adequate.
In this manner, methods for separating carbamic acid esters from a mixture obtained in a carbamic acid ester production process still have problems remaining.
On the other hand, various methods have been proposed for producing isocyanates by using a carbamic acid ester as a raw material.
According to the description of Patent Document 3, an aliphatic diurethane and/or alicyclic diurethane and/or aliphatic polyurethane and/or alicyclic polyurethane are obtained by reacting an aliphatic primary diamine and/or alicyclic primary diamine and/or aliphatic primary polyamine and/or alicyclic primary polyamine with O-alkylcarbamate in the presence of an alcohol and in the presence or absence of a catalyst at from 160 to 300° C. at a ratio of amine NH2 group: carbamate: alcohol of 1:0.8 to 10:0.25 to 50, followed by removing the ammonia formed as necessary. The resulting diurethane and/or polyurethane can be converted to the corresponding diisocyanate and/or highly functional polyisocyanate as necessary. Detailed reaction conditions with respect to thermal decomposition are not described in this publication.
According to Patent Document 4, an aromatic diisocyanate and/or polyisocyanate are produced by going through the following two steps. In the first step, an aromatic primary amine and/or aromatic primary polyamine are reacted with O-alkylcarbamate in the presence or absence of a catalyst and in the presence or absence of urea and alcohol to form an aryl diurethane and/or aryl polyurethane followed by removing the ammonia formed as necessary. In the second step, an aromatic isocyanate and/or aromatic polyisocyanate are obtained by thermal decomposition of the aryl diurethane and/or aryl polyurethane.
Several methods are known for forming a corresponding isocyanate and alcohol by thermal decomposition of a (cyclic) aliphatic, and particularly aromatic monourethane and diurethane, and although these methods may include methods carried out in a gaseous phase at a high temperature and methods carried out in a liquid phase under comparatively low temperature conditions, since there are cases in which the reaction mixture forms precipitates, polymeric substances and occlusions in the reaction vessel and recovery apparatus due to the occurrence of side reactions as described above, for example, economic efficiency is poor in the case of producing isocyanates over a long period of time.
Thus, chemical methods such as the use of a special catalyst (see Patent Document 15 and Patent Document 16) or a catalyst in combination with an inert solvent (see Patent Document 17) have been disclosed to improve yield during thermal decomposition of urethanes.
More specifically, Patent Document 18 describes a process for producing hexamethylene diisocyanate comprising thermal decomposition of hexamethylene diethylurethane in the presence of dibenzyl toluene used as a solvent, and in the presence of a catalyst mixture containing methyl toluenesulfonate and diphenyl tin dichloride. However, since there is no detailed description provided regarding production of the starting components or isolation, purification or voluntary recovery of the solvent and catalyst mixture, it is not possible to assess the economic efficiency of this process.
According to the method described in Patent Document 19, urethane can be easily decomposed to an isocyanate and an alcohol in a carbon-containing fluidized bed without using a catalyst. In addition, according to Patent Document 20, hexamethylene dialkyl urethane can be decomposed in a gaseous phase at a temperature in excess of 300° C. in the presence or absence of a gas-permeable packaging material composed of, for example, carbon, copper, bronze, steel, zinc, aluminum, titanium, chromium, cobalt or quartz to form hexamethylene diisocyanate. According to the description of Patent Document 14, the process is carried out in the presence of a hydrogen halide and/or hydrogen halide donor. However, this method is unable to achieve a hexamethylene diisocyanate yield of 90% or more. This is because the decomposition products are partially rebonded resulting in the formation of urethane bonds. Thus, further purification of hexamethylene diisocyanate by distillation is required, and this frequently results in an increase in yield loss.
Moreover, Patent Document 21 discloses that a monocarbamate can be decomposed at a satisfactory yield without using a solvent in the presence or absence of a catalyst and/or stabilizer advantageously under reduced pressure and at a comparatively low temperature. The decomposition products (monoisocyanate and alcohol) are removed from the boiling reaction mixture by distillation and are captured separately by fractional condensation. A method is described in a generic form for partially removing the reaction mixture in order to remove by-products formed during thermal decomposition. Thus, although by-products can be removed from the bottom of the reaction vessel, the problem with respect to the case of adhering to the walls of the reaction vessel as previously described remains, and the problem with respect to long-term operation is unresolved. In addition, there is no description regarding the industrial use of the removed residue (containing a large amount of useful components).
According to the description of Patent Document 22, thermal decomposition of an aliphatic, alicyclic or aromatic polycarbamate is carried at from 150 to 350° C. and from 0.001 to 20 bar in the presence of an inert solvent and in the presence or absence of a catalyst, auxiliary agent in the form of hydrogen chloride, organic acid chloride, alkylating agent or organic tin chloride. By-products formed can be removed continuously from the reaction vessel together with the reaction solution, for example, and a corresponding amount of fresh solvent or recovered solvent is added simultaneously. A disadvantage of this method is that, for example, a decrease in the space-time yield of polyisocyanate occurs due to the use of a refluxing solvent, and what is more, a large amount of energy is required, including that for recovering the solvent, for example. Moreover, the auxiliary agent that is used is volatile under the reaction conditions, and the decomposition products may be contaminated. In addition, the amount of residue is large relative to the amount of polyisocyanate formed, thereby making the economic efficiency and reliability as an industrial method suspect.
Patent Document 23 describes one method for continuous thermal decomposition of a carbamate, such as an alicyclic diurethane 5-(ethoxycarbonylamino)-1-(ethoxycarbonylaminomethyl)-1,3,3-trimethylcyclohexane, supplied along the inner surface of a tubular reaction vessel in a liquid form in the presence of a high boiling point solvent. This method has the disadvantages of low yield and low selectivity during production of a (cyclic) aliphatic diisocyanate. In addition, there is no description of a continuous method accompanying recovery of rebonded or partially decomposed carbamate, and post-treatment of solvent containing by-products and catalyst is also not mentioned.
The description of Patent Document 24 relates to a circulation method for producing (cyclic) aliphatic diisocyanate by converting a corresponding diamine to diurethane followed by thermal decomposition of this urethane. This method minimizes the decrease in yield by recirculating the product from a urethane decomposition step to an urethanation step following reaction with alcohol. By-products that are unable to be recirculated are removed by separation by distilling a mixture of urethanation products, and in this case, unwanted residue forms in the form of bottom products while all comparatively low boiling point components, including diurethane, are removed from the top of the column. This method, however has a disadvantage of using a large amount of energy. This is because all diurethane is required to be evaporated in the presence of a catalyst, and this diurethane must be evaporated at a temperature level within the range of the decomposition temperature of urethane. Isocyanate groups formed in useful products react with residual urethane groups, frequently resulting in the formation of comparatively high molecular weight by-products that cause a reduction in yield.
In addition, according to the description of Patent Document 25, a method is disclosed whereby unwanted by-products are partially removed prior to carrying out thermal decomposition of polyurethane. The disadvantage of this method is that the yield of isocyanate decreases as a result of polyurethane being contained in the partially removed by-products. In addition, since polymeric compounds form and adhere to the reaction vessel as a result of heating of by-products remaining in the reaction vessel without being discharged from the reaction vessel, long-term, continuous operation is difficult.
As has been described above, processes for producing isocyanates using carbamic acid esters as raw materials have numerous problems to be solved and have yet to be industrialized.    Patent Document 1: Japanese Patent Application Laid-open No. S59-48452    Patent Document 2: Japanese Patent Application Laid-open No. 2004-262831    Patent Document 3: U.S. Pat. No. 4,497,963    Patent Document 4: U.S. Pat. No. 4,290,970    Patent Document 5: U.S. Pat. No. 4,388,238)    Patent Document 6: U.S. Pat. No. 4,430,505)    Patent Document 7: U.S. Pat. No. 4,480,110    Patent Document 8: U.S. Pat. No. 4,596,678    Patent Document 9: U.S. Pat. No. 4,596,679    Patent Document 10: European Patent Laid-open No. 0320235    Patent Document 11: U.S. Pat. No. 4,395,565    Patent Document 12: Japanese Patent Application Laid-open No. H10-87598    Patent Document 13: Japanese Patent Application Laid-open No. 2001-48839    Patent Document 14: Japanese Patent Application Laid-open No. S64-85956    Patent Document 15: U.S. Pat. No. 2,692,275    Patent Document 16: U.S. Pat. No. 3,734,941    Patent Document 17: U.S. Pat. No. 4,081,472    Patent Document 18: U.S. Pat. No. 4,388,426    Patent Document 19: U.S. Pat. No. 4,482,499    Patent Document 20: U.S. Pat. No. 4,613,466    Patent Document 21: U.S. Pat. No. 4,386,033    Patent Document 22: U.S. Pat. No. 4,388,246    Patent Document 23: U.S. Pat. No. 4,692,550    Patent Document 24: European Patent No. 0355443    Patent Document 25: Japanese Patent No. 3382289    Non-Patent Document 1: Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870    Non-Patent Document 2: Journal of American Chemical Society, Vol. 81, p. 2138, 1959